Field of the Invention
This invention relates generally to the field of geological exploration for hydrocarbons. More specifically, the invention relates to an NMR sensor and a method of analyzing samples from a subsurface formation.
Background of the Invention
Nuclear magnetic resonance (NMR) is a powerful tool for analysis of rock core samples extracted from underground formations during oil and gas exploration and production. NMR is sensitive to water and hydrocarbons, but insensitive to the rock matrix, providing a porosity measurement that includes only the fluid in the pore spaces. It can also be used to characterize the mixture of fluids present in the sample as extracted, including both bound and free water, and different molecular weights of both live and dead oil. NMR also provides a non-destructive method for measuring the pore size distribution of the sample, unlike mercury injection methods which contaminate the sample.
NMR core measurements are conducted using “low-field” devices that typically apply a static magnetic field on the order of 500 Gauss (G) to the sample, yielding an NMR measurement frequency on the order of 2 MHz for hydrogen nuclei (similar to the measurement frequency of NMR logging tools). Such fields can be generated using arrays of permanent magnets. For comparison, “high-field” magnetic resonance devices common in medical and chemistry applications typically apply a static field in the range of 10,000 G to more than 200,000 G, for frequencies of 40 MHz to more than 1 GHz; it is generally not possible to achieve such field strength using permanent magnets, and so these fields can only be sustained using superconducting coils. (Electromagnets can be used for fields up to about 30,000 G.) Low-field measurements are preferred for rock analysis because they are more easily correlated with logging data, because low-field devices are significantly less expensive both to purchase and to maintain, and because strong magnetic fields introduce unacceptable susceptibility artifacts caused by distortions of the field at the surface of the rock matrix (such as at the rock-pore boundaries).
One problem with current devices is the lack of parallelism; samples are analyzed one at a time, and multiple scans (for purposes of signal averaging) generally require a low duty cycle because of the long repolarization time of the nuclear spins in the sample. It could potentially be much more efficient to use a device that can be loaded with multiple samples and analyze them in parallel.
Consequently, there is a need for improved methods and tools to use NMR for analyzing rock core or fluid samples from potential or existing hydrocarbon-bearing subsurface formations.